Advent-of-Code-2023/day16/floorWillBeLava.go

package day16

import (
	"fmt"
	"log"
	"math"
	"os"
	"strings"
	"sync"
)

func Run() int {
	fmt.Println("hello from day 16")
	log.Println("starting")
	filename := "day16/example"
	field := ReadField(filename)
	startPoints := StartPoints(&field)

	var startPointsWaitGroup sync.WaitGroup
	startPointsWaitGroup.Add(len(startPoints))

	results := make(chan int)

	go func() {
		startPointsWaitGroup.Wait()
		close(results)
	}()

	for _, start := range startPoints {
		go func(start MovementPoint) {
			cleanField := ReadField(filename)
			cleanField.StartTraversal(start)
			thisResult := cleanField.CountEnergized()
			results <- thisResult
			startPointsWaitGroup.Done()
		}(start)
	}

	max := math.MinInt
	for energized := range results {
		if energized > max {
			max = energized
			log.Println("found new max: ", max)
		}
	}

	// fmt.Println(field.String())
	// field.StartTraversal()

	// fmt.Println(field.ShowEnergyzed())

	return max
}

func StartPoints(f *Field) []MovementPoint {
	result := make([]MovementPoint, 0)

	for rowNum, row := range f.cells {
		result = append(result,
			MovementPoint{Row: rowNum, Col: 0, Direction: Rightward},
			MovementPoint{Row: rowNum, Col: len(row) - 1, Direction: Leftward})
	}

	for colNum, _ := range f.cells[0] {
		result = append(result,
			MovementPoint{Row: 0, Col: colNum, Direction: Downward},
			MovementPoint{Row: len(f.cells) - 1, Col: colNum, Direction: Upward})

	}

	return result
}

// have shared field
// running traversal recursive function per ray
// exit if going out of field or visiting cell that already had light in this direction
// (i.e encountering loop)

type CellType rune

const (
	Empty           CellType = '.'
	SplitterNS               = '|'
	SplitterEW               = '-'
	MirrorBackslash          = '\\'
	MirrorSlash              = '/'
)

type Direction int

const (
	Upward Direction = iota
	Downward
	Leftward
	Rightward
)

type Cell struct {
	CellType   CellType
	KnownBeams map[Direction]any
}

type Field struct {
	cells [][]*Cell
}

func (f *Field) isValid(mp MovementPoint) bool {
	if mp.Row < 0 || mp.Col < 0 {
		return false
	}
	if mp.Row >= len(f.cells) || len(f.cells) == 0 || mp.Col >= len(f.cells[0]) {
		return false
	}
	return true
}

func ReadField(filename string) Field {
	result := Field{}
	bytes, err := os.ReadFile(filename)
	if err != nil {
		panic(fmt.Sprint("cannot read file: ", filename))
	}
	text := string(bytes)
	text = strings.TrimSpace(text)
	for _, line := range strings.Split(text, "\n") {
		rowCells := make([]*Cell, 0)
		for _, symb := range line {
			rowCells = append(rowCells, &Cell{
				CellType:   CellType(symb),
				KnownBeams: make(map[Direction]any),
			})
		}
		result.cells = append(result.cells, rowCells)
	}
	return result
}

func (f *Field) String() string {
	result := "\n"
	for _, row := range f.cells {
		for _, cell := range row {
			result += string(cell.CellType)
		}
		result += "\n"
	}
	return result
}

func (f *Field) ShowEnergyzed() string {
	result := "\n"
	for _, row := range f.cells {
		for _, cell := range row {
			if len(cell.KnownBeams) > 0 {
				result += "#"
			} else {
				result += string(cell.CellType)
			}

		}
		result += "\n"
	}
	return result

}

type MovementPoint struct {
	Row, Col  int
	Direction Direction
}

func (f *Field) StartTraversal(startPoint MovementPoint) {
	reportedVisits := make(chan MovementPoint)
	var wg sync.WaitGroup

	go f.RecordVisits(reportedVisits)
	wg.Add(1)
	go f.TraverseFrom(startPoint, reportedVisits, &wg)

	wg.Wait()
	close(reportedVisits)
}

func (f *Field) CountEnergized() (result int) {
	for _, row := range f.cells {
		for _, cell := range row {
			if len(cell.KnownBeams) > 0 {
				result += 1
			}
		}
	}
	return
}

func (f *Field) RecordVisits(reportedPoints <-chan MovementPoint) {
	for point := range reportedPoints {
		cell := f.cells[point.Row][point.Col]
		log.Printf("recording visit %+v to %+v at row %d col %d\n", point, cell, point.Row, point.Col)
		cell.KnownBeams[point.Direction] = struct{}{}
	}
}

// starting at point, mark as visited
// move (concurrently if required) into next points
// ends - when out of the field, or if encountering a cycle
func (f *Field) TraverseFrom(current MovementPoint, reportVisits chan<- MovementPoint, wg *sync.WaitGroup) {
	log.Printf("> starting traverse through %+v", current)
	if !f.isValid(current) {
		log.Println("invalid current ", current, " should be impossible")
		wg.Done()
		return
	}
	cell := f.cells[current.Row][current.Col]
	_, knownDirection := cell.KnownBeams[current.Direction]
	if knownDirection {
		log.Printf("found cycle at %+v in %+v", current, cell)
		wg.Done()
		return
	}

	reportVisits <- current

	nextPoints := NextPoints(f, current)
	log.Printf("for current %+v next are: %+v\n", current, nextPoints)
	switch len(nextPoints) {
	case 0:
		wg.Done()
		return
	case 1:
		f.TraverseFrom(nextPoints[0], reportVisits, wg)
		return
	case 2:
		wg.Add(1)
		go f.TraverseFrom(nextPoints[0], reportVisits, wg)
		f.TraverseFrom(nextPoints[1], reportVisits, wg)
		return
	}

}

func NextPoints(f *Field, current MovementPoint) []MovementPoint {
	cell := f.cells[current.Row][current.Col]
	nextDirections := cell.CellType.NextDirections(current.Direction)
	nextCells := make([]MovementPoint, 0)
	for _, direction := range nextDirections {
		nextMovementPoint := current.ApplyDirection(direction)
		if f.isValid(nextMovementPoint) {
			nextCells = append(nextCells, nextMovementPoint)
		}
	}
	return nextCells
}

// value receiver, can safely modify incoming mp
// doesn't know about Field dimentions
func (mp MovementPoint) ApplyDirection(d Direction) MovementPoint {
	switch d {
	case Upward:
		mp.Row -= 1
	case Downward:
		mp.Row += 1
	case Leftward:
		mp.Col -= 1
	case Rightward:
		mp.Col += 1
	}
	mp.Direction = d
	return mp
}

func (ct CellType) NextDirections(currentDirection Direction) (nextDirections []Direction) {
	switch ct {
	case Empty:
		nextDirections = []Direction{currentDirection}
	case SplitterNS:
		if currentDirection == Rightward || currentDirection == Leftward {
			nextDirections = []Direction{Upward, Downward}
		} else {
			nextDirections = []Direction{currentDirection}
		}
	case SplitterEW:
		if currentDirection == Downward || currentDirection == Upward {
			nextDirections = []Direction{Leftward, Rightward}
		} else {
			nextDirections = []Direction{currentDirection}
		}
	case MirrorBackslash:
		// mirror symbol is \
		directionMappings := map[Direction]Direction{
			Leftward:  Upward,
			Rightward: Downward,
			Upward:    Leftward,
			Downward:  Rightward,
		}
		nextDirections = []Direction{directionMappings[currentDirection]}
	case MirrorSlash:
		// mirrow symbol is /
		directionMappings := map[Direction]Direction{
			Leftward:  Downward,
			Rightward: Upward,
			Upward:    Rightward,
			Downward:  Leftward,
		}
		nextDirections = []Direction{directionMappings[currentDirection]}
	}
	return
}